When a printer or copier does two-sided copying, it generally prints one side of the paper first, then flips the paper over and prints the other side. A large number of patents and published patent applications exist for paper-flipping mechanisms, including GB 2 168 688 A to Xerox Corp., U.S. Pat. No. 4,969,641 to Fukushima et al., U.S. Pat. No. 5,201,517 to Stemmle, U.S. Pat. No. 5,692,740 to Holtje, U.S. Pat. No. 5,692,747 to Guerrero et al., U.S. Pat. No. 5,720,478 to Carter et al., U.S. Pat. No. 6,199,858 to Wyer, and PCT publication WO 02/060794 A1 to Hallmark Cards Inc. These mechanisms typically work by moving the paper into an intermediate position, and then moving it back out along a different path, so that its orientation is changed. To prevent the paper from going back along the same path when its motion is reversed, something irreversible is done to it before moving it back. In some publications, the irreversible action is falling or sliding under the influence of gravity. Using gravity to move the paper reduces the need for active mechanisms such as motor-driven rollers or pneumatics, reducing cost and increasing reliability. However, the falling or sliding paper must be guided fairly closely to make sure it ends up in the right place without wrinkling or jamming. This leads to the possibility that the first printed side of the paper will be scratched or smudged as a result of rubbing against a surface that is guiding it.
WO 02/060794 A1 discloses a mechanism for flipping card stock for two-sided printing, in which the surface is never rubbed. The card stock is held rigidly in a frame, and the whole frame is flipped over.
U.S. Pat. No. 6,199,858 to Wyer illustrates the difficulty of designing a flipping mechanism using rollers, that does not allow the paper to rub. The device disclosed in Wyer has the primary purpose of forming a shingled stack of paper, but it also flips the paper when forming the stack. In this device, the paper is fed through a first pair of rollers, and the lead end drops straight down to an intermediate position where it is caught between a second pair of rollers. If the paper were very compliant and with no memory of its curvature, then there would be no need for guide walls, and the paper would drop directly between the second pair of rollers without rubbing against anything. But since paper is not completely compliant and does retain a curl, guide walls are needed to make sure the leading edge of the paper is fed into the gap between the second pair of rollers, and the paper can rub against the guide walls.
Other mechanisms for flipping print media, too numerous to mention, are also known in the art.